Add a Sphinx extension to document cached properties of slots classes #1967

Job	Run time
Build & verify package	18s
Verify dev env	42s
Check types using supported type checkers	42s
Tests on pypy-3.10	14s
Tests & Mypy on 3.14	15s
Run doctests & render changelog	15s
Tests & Mypy on 3.9	18s
Tests & Mypy on 3.11	9s
Tests & Mypy on 3.13	11s
Tests & Mypy on 3.12	14s
Tests & Mypy on 3.10	12s
Combine & check coverage	0s
required-checks-pass	3s
	3m 33s

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